1. Field of the Invention
The present invention relates to a method of estimating a surface temperature of a substrate during epitaxial growth of a semiconductor layer on the substrate in a semiconductor device fabrication process.
2. Description of the Related Art
In recent years, there have been developed high-performance semiconductor devices so-called heterostructure devices such as hetero bipolar transistors (hereinafter referred to as “HBTs”). Such a heterostructure device is fabricated by epitaxial growth of a crystal that is different from the substrate. The use of such epitaxial growth technology makes it possible to stack thin crystal films that are different in physical properties from each other with a high precision of the order of about a mono atomic layer to several nanometers.
In the fabrication of such a heterostructure device, control of the film thickness and composition ratio profile of an epitaxial layer is critical. This is because, without such control, volume production of heterostructure devices exhibiting desired characteristics would be impossible. The thickness and composition profile of the epitaxial layer are determined by epitaxial growth conditions. Therefore, in volume production of heterostructure devices, it is necessary to reproduce growth conditions that have been previously confirmed to provide such devices with desired characteristics.
Conventionally, the CVD (Chemical Vapor Deposition) process has been widely used. In this process, epitaxial growth is performed by supplying a vapor (source gas) as a material for growth into a growth chamber in which a heated substrate is placed to cause a chemical reaction to occur between the source material molecule and the material of the substrate on the substrate surface. In the case of the CVD process, the composition ratio of a crystalline thin film of a mixed crystal material such as SiGe (silicon germanium) can be controlled by adjusting the mixing ratio between plural kinds of source gases to be supplied.
The CVD process causes epitaxial growth to proceed by the chemical reaction between the material of the substrate on the substrate surface and the source material molecule as described above and, hence, the rate of growth is susceptible to the temperature of the portion where reaction proceeds. In the case of the UHV-CVD (Ultra High Vacuum Chemical Vapor Deposition) process employed for epitaxial growth of an SiGe mixed crystal, in particular, the rate of growth is susceptible to the surface temperature of the substrate. The UHV-CVD process referred to herein is a technique enabling crystal growth to proceed at temperatures as low as about 500° C. to about 600° C. by providing an ultra-high vacuum atmosphere in the crystal growth chamber. Since the crystal growth proceeds under such a low temperature, the UHV-CVD process allows a condition called “reaction control condition” in which the rate of growth is completely controlled by the speed at which the chemical reaction proceeds at the substrate surface and hence hardly depends on the amounts of the source gases supplied. Under the reaction control condition, the rate of growth depends substantially only on the surface temperature of the substrate and hence is sensitive to variation in the surface temperature of the substrate. For instance, where silicon is epitaxially grown on a substrate heated to a surface temperature of about 600° C. by the UHV-CVD process, a variation of only 1° C. in the substrate surface temperature causes the rate of growth to vary as largely as about 3%.
Therefore, in volume production of heterostructure devices by the UHV-CVD process, the substrate temperature is most important among the growth conditions to be reproduced. This is because even if the growth conditions other than the substrate temperature, such as the gas flow rate and growth time are reproduced precisely, a failure to reproduce the substrate temperature results in a grown layer having a film thickness and a composition ratio profile that are different from desired ones, thus making it impossible to fabricate heterostructure devices having desired characteristics.
In volume production of heterostructure devices, usually, a plurality of devices are fabricated on a single substrate and a required number of substrates are subjected to processing. Therefore, in order to fabricate a required number of heterostructure devices with a high yield, making each of the growth conditions uniform in the plane of the substrate and making the growth conditions constant for every substrate are both necessary. Accordingly, it is important in the UHV-CVD process to make the temperature uniformity in the substrate plane higher and make the substrate temperature constant for every substrate.
In reproducing the surface temperature of a substrate, a technique of measuring the surface temperature is needed. That is, if the temperature distribution in the plane of a substrate under epitaxial growth cannot be determined, adjustment to make the temperature in the plane of the substrate uniform is impossible. Further, if the temperatures of respective substrates cannot be measured, it is impossible to reproduce a desired substrate surface temperature.
In semiconductor device fabrication processes, a thermocouple and a pyrometer have been conventionally widely used, in a technique for measuring the surface temperature of a substrate.
The thermocouple is constructed of two kinds of metals joined together and is adapted to measure a temperature by measuring a voltage generated in accordance with a difference in thermoelectromotive force between the metals when the temperature of the joint between the two metals reaches a predetermined value.
However, the thermocouple needs to be placed close to a subject of measurement, i.e. a substrate surface because of its operating principle and hence is not suitable for measurement of the temperature of a substrate surface on which a chemical reaction is proceeding. For this reason, the thermocouple cannot measure the temperature of a part other than the substrate surface such as a heater or the inside of a susceptor and hence is incapable of precise measurement of the temperature of the substrate surface. Further, since the joint between the metals of the thermocouple needs to be connected to a voltmeter through electric wire, multipoint measurement for measuring the temperature distribution in the plane of a substrate requires many interconnecting wires, so that the number of measurement points is limited.
On the other hand, the pyrometer is capable of measuring the temperature of a substrate surface on which a chemical reaction is proceeding since the pyrometer is adapted to measure a temperature by measuring the intensity of electromagnetic wave emitted from a measuring region. To determine an exact temperature from an electromagnetic wave intensity, the physical quantity called emissivity needs to be known. Actually, however, there is a difficulty in measuring the emissivity, which makes it difficult for the pyrometer to measure an exact temperature. Particularly, the emissivity of a substrate provided with a pattern depends greatly on the pattern, the material and thickness of the substrate, and the like. Further, the emissivity of such a substrate provided with the pattern varies with epitaxial growth. For this reason, it is, in effect, impossible to measure the surface temperature of a substrate provided with the pattern with use of the pyrometer during epitaxial growth.
It is, therefore, a conventional practice to control the temperature of the substrate provided with the pattern during epitaxial growth by measuring the temperature of the part other than the substrate surface such as the susceptor and controlling the heater based on the temperature thus measured. Such a conventional practice, however, involves a problem that even though the temperature of the susceptor for example is adjusted to a target temperature, the temperature of the substrate surface does not become equal to the target temperature because the influence of the pattern made of an oxide film and polysilicon on the substrate causes the emissivity, thermal conductivity, heat capacity and the like of the substrate to vary.
In measuring a substrate surface temperature with the pyrometer, a growth apparatus needs to have a window called view port for allowing the substrate surface to be observed. Such a view port raises another problem that the number of measurement points is usually limited to a few points in spite of the need of temperature measurement at many measurement points for measurement of the temperature distribution in the plane of the substrate.
The present invention has been made in view of the foregoing circumstances. It is therefore an object of the present invention to provide a method of estimating substrate temperature which is capable of estimating the surface temperature of a substrate provided with a pattern during epitaxial growth.
Another object of the present invention is to provide a method of estimating substrate temperature which is capable of estimating the temperature distribution in a plane of a substrate by estimating temperatures at plural points in the plane of the substrate.